It is known that the titanium dioxide exhibits a change in electrical resistivity when exposed to a change in oxygen partial pressure. An especially big change in electrical resistivity occurs when the atomsphere changes from oxidizing to reducing. For example, when titanium dioxide is exposed to equilibrated exhaust gases resulting from combustion of a fuel lean air/fuel mixture, the titanium dioxide will exhibit a high resistivity. When exposed to equilibrated exhaust gases resulting from combustion of a fuel rich air/fuel mixture, the titanium dioxide will exhibit a significantly lower resistivity. For this reason, titanium dioxide has previously been used in internal combustion engine exhaust gas oxygen sensors to detect departures in exhaust gas composition from stoichiometry. In sensing such departures, of course, one is indirectly sensing engine intake air/fuel ratio. Such sensing can be used, for example, to regulate a closed loop air/fuel ratio control system for an internal combustion engine.
A complicating factor is that titanium dioxide resistivity also changes as a function of temperature. Further, internal combustion engine exhaust gas temperatures vary widely. Further, the exhaust gases should be equilibrated, or inaccuracies result. Porous titanium diode materials impregnated with oxidation catalysts have been used to reduce such inaccuracies. Nonetheless, at low temperatures resistivity has increased too much. In addition, the more porour bodies used to obtain more rapid equilibration may be relatively soft and questionably durable. Increasing hardness may have required high sintering temperature and corresponding losses in surface area, adversely affecting low temperature performance.
Aggravating the aforementioned problems, is that the amplitude of the exhaust gas temperature variation is increasing. Exhaust gas temperatures in some more recent fuel efficient engines can now stay at much lower levels, both on engine warm-up and at idle after warm-up. There is no titanium dioxide exhaust gas oxygen sensor publically disclosed or commercially available that can be used with such engines without requiring some form of temperature compensation or supplementary heating. Temperature compensation alone may not even be enough, if sensor cycle time at the low temperatures is too slow for the system in which it is used. In such instance, the sensor simply does not control the system at low temperatures. Prior sensors require temperature compensation and/or have controllability problems below 300.degree. C.
I have found how to provide a titanium dioxide sensor that has a sufficiently low resistance at low temperatures that neither temperature compensation nor supplementary heating is required. In addition, I have found how to make the sensor body more durable by a technique in which severe firing is unnecessary. Cycle time is comparable to an existing sensor and response time may even be quicker.